Fibrous micro-composite material

ABSTRACT

Fibrous micro-composite materials are formed from micro fibers. The fibrous micro-composite materials are utilized as the basis for a new class of MEMS. In addition to simple fiber composites and microlaminates, fibrous hollow and/or solid braids, can be used in structures where motion and restoring forces result from deflections involving torsion, plate bending and tensioned string or membrane motion. In one embodiment, fibrous elements are formed using high strength, micron and smaller scale fibers, such as carbon/graphite fibers, carbon nanotubes, fibrous single or multi-ply graphene sheets, or other materials having similar structural configurations. Cantilever beams and torsional elements are formed from the micro-composite materials in some embodiments.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/500,011, filed Aug. 7, 2006, now U.S. Pat. No. 7,675,698, which is acontinuation of U.S. patent application Ser. No. 10/395,008, filed Mar.21, 2003, now U.S. Pat. No. 7,405,854, which claims priority to U.S.Provisional patent application Ser. No. 60/366,454, filed Mar. 21, 2002,which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to composite materials, and in particularto fibrous micro-composite materials

BACKGROUND OF THE INVENTION

Present day micro-electro-mechanical systems (MEMS) based actuatordevices have fundamental performance issues that severely limit theirwidespread commercialization. Although MEMS manufacturers have pushed todevelop silicon and other material-based structures, the resultingsystems still lack the needed mechanical properties. A specific exampleis the case of MEMS based optical scanner and switches (OMEMS). Suchdevices need to produce large angular deflections (several tens ofdegrees) and resonant frequencies exceeding tens of kilohertz withlifetime reliability over billions of cycles.

Monolithic materials, such as silicon, metal and ceramic thin filmscurrently used to produce MEMS lack the required combination of highelastic stiffness, high strength, high fatigue lifetime and low density(mass per unit volume) i.e., the basic mechanical flexibility and flawtolerance necessary for many potential MEMS applications. Polymers, arenot adequate since they are too flexible and have low strength whichlimits them to low frequency operation in devices where low forcesand/or displacements are required, such as valves and fluidic pumps.

Consequently, moving component MEMS, such as optical scanners, arenearly non-existent commercially today. Most successful applications ofMEMS remain based on quasi-static devices such as pressure andacceleration sensors. One moving component MEMS is a digital lightprocessor that is based on bistable positioning of aluminum MEMSmirrors.

The need for advanced capability MEMS devices can be illustrated througha particular application—the MEMS based optical scanner (an OMEMS). Suchscanners are envisioned for large area display applications usingthree-color scanning. Early MEMS optical scanners utilized a torsionalsilicon micro-mirror produced using wet etching. It was capable ofdeflecting a beam through a 0.8° angle at a resonance frequency of 16.3kHz. The majority of OMEMS scanners in development today are stilldesigned using similar thin beams of silicon acting either as torsionbars (around which a silicon mirror element rotates) or as cantilevers(which vibrate to provide the scanning motion). Both of these structuretypes are efficient, with no moving parts to wear.

General applications are dependent on the resonance frequency, themaximum deflection, and the maximum restoring force—with higher valuesof each normally desired. These properties are dependent on the size,shape, and mechanical properties of the underlying materials. However,materials used in traditional IC-based MEMS fabrication lack themechanical characteristics required to allow specific tailoring andoptimization for many applications. There is no current way to designsimultaneously for high frequency operation, large amplitude deflection,low operating power, robustness, and long-term reliability under cyclicstresses with existing material systems. The basic problem with silicon,and monolithic materials in general, is that while having sufficientelastic stiffness, their strength and fatigue lifetime is too low anddensity too high. This combination limits the ultimate deflectionamplitude and frequency, and increases power requirements to sustainoscillation.

SUMMARY OF THE INVENTION

Fibrous micro-composite materials are formed from micro fibers. Thefibrous micro-composite materials are utilized as the basis for a newclass of MEMS. In addition to simple fiber composites andmicrolaminates, fibrous hollow and/or solid braids, can be used instructures where motion and restoring forces result from deflectionsinvolving torsion, plate bending and tensioned string or membranemotion. In some embodiments, these materials will enable simultaneoushigh operating frequencies, large amplitude displacements and orrotations, high reliability under cyclical stresses.

In one embodiment, fibrous elements are formed using high strength,micron and smaller scale fibers, such as carbon/graphite fibers, carbonnanotubes, fibrous single or multi-ply graphene sheets, or othermaterials having similar structural configurations.

In a further embodiment, cantilever beams are fabricated from singlefibers, single/multilayer aligned arrays of fibers, or single/multilayerfabrics. Such fabrics exploit the special strong anisotropic mechanicalproperties and high strength along the fiber axis of the fibers yieldingstructures with high bending stiffness, and low mass, yet large bendingcurvatures. Single fiber cantilevers provide the highest possibleoperating frequencies for potential applications such as RF sensors, atthe expense of lateral stiffness and strength. Multifiber cantileversbenefit from statistical improvements and stability based on averagingproperties and load sharing in the event of fiber damage or intrinsicfaults. The natural extension is to more complex fabrics with optimizedproperties in multiple directions or multiple modes of deflection. Suchcantilevers can also be produced from braided torsion elements,producing both lateral and angular displacements.

In still further embodiments, plates (two dimensional minimallydeformable objects) are fabricated from single/multilayer aligned arraysof fibers, or single/multilayer fabrics. This configuration optimizesthe stiffness to mass ratio together with the strength required for highfrequency motion, such as required for the mirror element in a scannerMEMS. Relative stiffness in the two axes may be tailored to balancedriving forces through fiber density, type, orientation, positioningand/or weave characteristics.

In yet a still further embodiment, hollow or tubular micro-braids madefrom micron-scale fibers are used as torsional deflecting elements indevices to provide high performance MEMS actuators. Braids permit thetransformation of stresses within the torsion bar from shear (resultingfrom twisting motion) to tensile/compressive stresses (with somebending) along the orthogonal fiber axes at plus/minus 45 degrees. Ineffect, the braid allows the fibers to act in a mode in which theirbehavior is exceptional. Torsion elements at the sub 100 micron scale(comparable to MEMS device dimensions) can be fabricated from fibers 5microns in diameter; smaller fibers produce commensurately smallerbraids.

Additionally by manipulating the dimensions of the braid, the braidingangle, the types of fibers used to construct the braid, and the size andnumber of fibers in the braid, torsional elements with tailoredstrength, elastic stiffness, density, and other mechanical propertiescan be produced. This enhancement and tailorability of the strength andelastic stiffness of the torsional element results in MEMS devicescapable of producing large angular deflections and forces at highfrequencies and speeds without failure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a single micro-fiber encapsulated in amatrix supported by a substrate.

FIG. 2 is a cross section of a collection of parallel aligned fibersencapsulated in a matrix supported by a substrate.

FIG. 3 is a perspective view of multiple variations of micro-fiber crosssections.

FIG. 4 is a cross section view of a collection of multiple layers ofparallel aligned fibers encapsulated in a matrix supported by asubstrate.

FIG. 5 is a perspective schematic view of layers of bidirectionalmicro-fibers.

FIG. 6 is a perspective schematic view of a micro-fiber weaved fabrichaving a braid therein.

FIG. 7 is a perspective view of a multi micro-fiber cantilever supportedby a substrate.

FIG. 8 is a perspective view of a single micro-fiber cantileversupported by a substrate.

FIG. 9 is a perspective view of an alternative multi micro-fibercantilever supported by a substrate.

FIGS. 10A, 10B, 10C, 10D, 10E, 10F, 10G, 10H, 10I, 10J and 10K are crosssection views illustrating a process of forming cantilever MEMS devices.

FIG. 11 is a perspective view of a solid micro-fiber braid.

FIG. 12 is a perspective view of a hollow micro-fiber braid.

FIG. 13 is a perspective view of a solid low angle micro-fiber braid.

FIG. 14 is a schematic view of a scanner having nested frames and amirror supported by orthogonal pairs of fiber braids.

FIG. 15 is a cross section of a braid having a piezoelectric core.

FIG. 16 illustrates pre-etching of slots in a substrate to align fiberbraids during the process of forming the scanner of FIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the scope of the present invention. The following description is,therefore, not to be taken in a limited sense, and the scope of thepresent invention is defined by the appended claims.

Fibrous micro-composite materials are formed from micro fibers. A newclass of MEMS (Micro-Electro-Mechanical-System) devices is based on thefibrous micro-composite materials. Such fibrous micro-composite MEMSpromise to meet the requirements of high performance applications wherelarge deflections and forces, coupled with high operating frequency andreliability under cyclical stresses are required, and which cannot bemet by silicon and other currently used MEMS materials.

MEMS devices are constructed from a single or multi-fiber structureswith approximately 10 micron to sub-micron fibers, fiber braids, orfabrics. The fiber may be all of the same type (homogeneous) or ofdifferent types fabricated side-by-side or together (heterogeneous). Inone embodiment, the majority of the fibers are used to control themechanical properties of the MEMS device. A minority of fibers may beincorporated to provide other electrical, mechanical, biological, oroptical functionality. These may then be used, for example, toelectro-mechanically actuate the MEMS device or serve as associatedcontrol circuitry and sensing elements within the structures. The entiremicro-composite is impregnated with an appropriate binder (epoxy, glass,organic binders, etc.) to constrain the fibers and providefiber-to-fiber load transfer. The binder is referred to as a matrix. Inone embodiment, the matrix is chemically compatible with the selectedfibers, processing techniques, and the environment in which the MEMSdevice will operate.

MEMS are inherently micron-scale devices. Typical devices require afootprint of at most a few mm on an edge, and have thicknesses on theorder of a few tens or hundreds of microns. High strength fibers arealready commercially available in low to sub-micron diameters. Thesefibers can be readily oriented in any desired manner, and/or woven intofabrics that remain within the 100-200 μm range.

In FIG. 1, a suitable substrate 100, such as a silicon wafer is used tosupport a fiber 101 which is mounted in a suitable matrix 102. Thesingle fiber 101 is placed on the wafer 100 surface. The matrix 102encapsulates the fiber on the wafer surface. It is formed by spincoating or any other suitable method such as one that is compatible withprocesses common to MEMS and semiconductor fabrication. The spin coatingforms a film encapsulating the fiber on the wafer surface that is thencured thermally, or by using light or other radiation to form acomposite film.

In FIG. 2, a substrate 200 is shown supporting multiple fibers, 201,202, 203, 204 and 205 laid out substantially parallel and adjacent toeach other. A matrix 210 encapsulates the fibers to provide a singlelayer array of fibers.

Carbon/graphite fibers are the strongest material currently availablefor forming fibrous based fabrics and braids, and are extremely fatigueresistant in tension and compression even at high stress levels. Fibermaterial tensile strengths in excess of 20 GPa have been seen in bendingloop tests. Additionally, fiber strength is size dependent, increasingwhen fiber lengths decrease to sub millimeter levels or when fiberdiameter continually decreases below 10 microns. Micro-fibers also varybetween 2 cm to sub mm in length in one embodiment. Longer fibers mayalso be used.

Graphite fiber which has a mean tensile strength of 5 GPa at a 2 cmlength scale can have a tensile strength of 10 GPa at sub mm lengthscales and this can be “guaranteed” by selection through proof-testing.Thus, graphite fibers can be selected to have failure strainsconsistently above 5%. This is especially possible with the latestgeneration of graphite fibers reportedly with strengths at several cmgage lengths of 8 GPa so this should scale to 15 GPa at sub mmdimensions.

Graphite fibers exhibit the highest flaw tolerance of any material. Thestacked and folded graphene “sheet” nature of graphite fibers isolatesthe interior core from flaws on the outer surface of the fibers; fibersshed outer sheaths without failure of the entire fiber. Fibers also tendto increase their strength as the size is reduced—for carbon fibers, thehighest failure stresses (>15 GPa) are observed in 4-5 μm fibers. In afurther embodiment, it is desired that the fibers are comprised ofmultiple parallel carbon nanotubes. Once available, such fibers willallow the use of nanotubes in fiber bundles on the micrometer tomillimeter scale.

To ensure this high strength of fibers within the braid, 2-5 μm ToraycaT1000 carbon fibers which have stated strengths of approximately 1.2 Msi(˜8.3 GPa) at 2 to 3 cm gage lengths are used in one embodiment. Onlycarbon fibers that have been proof tested at over 5% strains are used tomake braids in one embodiment. Of course, other fibers may also be usedwith varying results.

Multiple different cross sections of fibers are shown in FIG. 3. Atypical round cross section fiber is indicated at 310. An ellipticalfiber cross section is shown at 315. A rectangular fiber cross sectionis shown at 320. At 325, a fiber cross section that is fairlyrectangular, but with rounded edges is shown. A grooved fiber crosssection is shown at 330, and a dogbone type cross section fiber is shownat 335. Each of these fibers may be used to create fibrousmicro-composite structures.

The matrix choice is important to achieving the desired mechanicalproperties of the micro-composite. In one embodiment, the matrix isdurable enough to survive a large number of deformation cycles, whereinit reliably transfers the load across filaments in the composite. Italso adheres strongly to outer packaging structures of the device, andserves as an efficient load transfer medium at the fiber terminations.It may also be compatible with deposition, etching etc., techniques andprocesses used in making MEMS devices.

Electro-active polymers may be utilized for the matrix. Piezoelectricpolymers may also be used as part of the matrix. Blends of differentmaterials may be used as matrices to obtain tailored electro-mechanicalproperties.

Methods to enhance fiber/matrix bonding, well known to a personexperienced in the art, can be similarly utilized in these structures.For example, matrix load-transfer characteristics may be modified byadjusting the sizing applied to different fibers. Requirements in themirror, frame, and flexure elements of MEMS scanners are different andsignificant alterations in surface treatments may be advantageous.

In addition to fibers that act as enhance the mechanical properties ofthe MEMS fibers ofelectro-chemically/mechanically/optically/biologically functionalmaterials may also be incorporated into said MEMS structures and devicesto act as actuating and or sensing elements. For example, magneticactuation is known to generate, at low power, the high forces requiredfor high speed and deflection. The strength of the interaction dependson the total volume of magnetic material that can be incorporated intothe MEMS structure. Traditionally, these are electroplated ontostructures of the MEMS device. In the case of fabrics, fibers offerromagnetic material, are woven directly into the fabric, integratingthe actuation directly into the overall fabric. Additionally,incorporating other types of fibers into the weave and creating a hybridfibrous structure can also create MEMS devices with integrated sensorelements.

The MEMS structures can also be made by layering arrays of orientedfibers 410, 415 and 420 in FIG. 4 in different configurations. Sucharrays are impregnated with an appropriate matrix 430 and converted intoa stiff composite. Or, by weaving filaments into a fabric. A fabric baseprovides advantages over laminated unidirectional fiber arrays since thefiber interlacing points provide additional stiffness and bettertransverse properties that help maintain mirror flatness duringoperation. Alignment of the fabric to specific elements or structureswithin the MEMS is also simpler. The weave of the fabric (plain, twill,satin, double, etc.) may be selected based on the desired stiffness ofthe mirror element. In this composite, the matrix serves primarily asload transfer between the fibers at low shear stresses and strains;though in the braid it will also serve to maintain mechanical stability.A wide range of matrices may be used and selected to achieve the desiredstiffness. This fabric-based (fibrous) composite, bonded to conventionalsilicon substrates, forms a base for the optical scanner, andpotentially for a wide variety of other MEMS structures.

The fibers, matrix, weave and braid characteristic can each beindependently optimized to achieve specific requirements. The fibertype, alignment/orientation, weave, and fiber volume fraction determinethe tensile and fracture strengths of micro-composites, as well as theirelastic stiffness. The stiffness of torsion elements formed from themicro-composites is dominated by the fiber axial stiffness incombination with curvatures associated with the braid characteristics,and to a much lesser extent the matrix modulus and the adhesion betweenthe fibers in the braid and the matrix. The fibers in the braids andfabric should be well adhered. Individual matrix areas may also bemodified separately, using localized cross-linking techniques, toachieve optimal mechanical properties within a MEMS device such as ascanner.

In this composite, the matrix serves primarily as load transfer betweenthe fibers A wide range of matrix material can be selected to achievethe desired behavior from the fibrous composite MEMS. This fabric-based(fibrous) composite, bonded to conventional silicon substrates, forms abase for the optical scanner, and potentially for a wide variety ofother MEMS structures.

In FIG. 5, adjacent layers of fibers are shown in a non-weaved pattern.A first layer 510 is comprised of a group of parallel adjacent fibersarranged in one plane, with an adjacent second layer 515 of a group ofparallel adjacent fibers arranged in a plane parallel to the first layer510. In one embodiment, the fibers in the two layers are substantiallyorthogonal to each other. The actual angle formed between the fibers ofthe two layers may be significantly varied from orthogonal to parallelas desired. Still further layers may be added with the fibers arrangedin the same manner or different manners, such as various weave patterns.

A variety of weaving and braiding patterns are possible to obtain thenecessary mechanical properties for the mirror and frame elements. Aplain weave as shown in FIG. 6 has the advantage of the highestinterlacing density, yielding a stiff composite for the mirror elementand frame. In FIG. 6, the woven fabric itself is a micro-compositeconstructed from several, micron to sub-micron scale, fibers, as shownat 600. The majority of the fibers (indicated by solid lines 610) arechosen to control the stiffness and strength (i.e. graphite, glass,nicalon) of the fabric, while some, such as a minor fraction of thefibers (indicated by dotted line 615) are ferromagnetic (i.e. nickel orPermalloy) to couple to a solenoid for actuation. Torsion bars may alsobe formed of a fiber braid 220 and integrated into the weave.

A satin weave has the least interlacing density, but permits a muchgreater fiber/yarn density in the fabric, increasing the fiber contentin the final composite. Twill weave interlacing density lies between thetwo and will depend on the type of the twill weave. Different weavepatterns may be tested to determine those providing optimal stiffnesswith relatively simple fabrication and processing.

One example MEMS device utilizing such fibrous materials of the presentinvention is a simple oscillating multiple fiber cantilever, such asthat shown in FIG. 7 at 700. In this embodiment, a matrix 710encapsulating multiple fibers 715 is attached to a substrate 720. Thefibers 715 and portion of the matrix 710 extends beyond the substrate720 as indicated at 730. The length of the extension and properties ofthe fiber and matrix determine the oscillatory characteristics of thecantilever 700. Further layers may be added, such as Al or Au may formedon top of the matrix.

One example MEMS device utilizing such fibrous materials of the presentinvention is a simple oscillating single fiber cantilever, such as thatshown in FIG. 8 at 800. In this embodiment, a matrix 810 encapsulating asingle fiber 815 is attached to a substrate 820. The fiber 815 andportion of the matrix 810 extends beyond the substrate 820 as indicatedat 830. The length of the extension and properties of the fiber andmatrix determine the oscillatory characteristics of the cantilever 800.In a further embodiment, a further layer 840 is formed on top of thematrix, at least in a footprint corresponding to the substrate 820. Suchfurther material comprises Au or Al in one embodiment, and it may becoupled as by gluing, or formed in many different ways known in thesemiconductor processing art.

In an example of a MEMS device having a multiple fiber cantilever 900 inFIG. 9, several fibers are encapsulated in a matrix 915. The matrix 915is coupled to a substrate 920. The fibers and matrix extend from an edgeof the substrate 920 as indicated at 940. The fibers 910 and matrix 915may be cut or trimmed as desired to obtain desired characteristics.

An example method for fabricating the cantilever MEMS structure of FIG.9 is shown in FIGS. 10A through 10L. In one embodiment single graphitefibers are extracted from bundles and placed side-by-side to form afiber array 1001 in FIG. 10A across the surface of an oxidized 1002silicon wafer 1003. The ends of the fiber arrays are glued to keep thefibers aligned with the substrate and with each other. The wafer withthe unidirectionally aligned fiber arrays is then coated with a suitablematrix 1004 such as polyimide, by spin coating the liquid matrixmaterial onto the substrate wafer 1003 to encapsulate the fiber array1001. Once a desired thickness of the matrix is achieved on thesubstrate and the fibers are encapsulated, the matrix is cured in afurnace at 350-400 degrees C.

A back surface 1010 of the substrate wafer (i.e., the surface withoutany fiber encapsulated in polyimide) as shown in FIG. 10B is then maskedfor a trench etch. The mask is selectively exposed 1012 such as by UVlight, and etched using wet etch chemistries common in CMOS processing.Specifically HF as seen in FIG. 10C is used to remove the oxide.Tungsten 1015 is blanket deposited to about 1000 Angstrom to protect thepolyimide on the front of the substrate in FIG. 10D. KOH is used toproduce trenches 1020 over which cantilever tips can freely oscillate asseen in FIG. 10E. The tungsten 1015 is then removed such as by using HFin FIG. 10F.

In FIG. 10G, a second mask 1023 is used to pattern cantilevers. Ablanket of Al 1024 is deposited on the wafer and lifted off in areasother than the cantilevers. The wafer containing the polyimideencapsulated fibers is then exposed and etched in a CF₄/O₂ plasma asshown in FIG. 10I using the Al as a self-aligned mask to remove polymerand oxide. This process produces rectangular beams of polyimideencapsulated fibers 1030 that cantilever over the edges of the trenchesetched into the substrate wafer from the back side as shown in side andtop views of FIGS. 10J and 10K respectively. A further polyimide film1035 may be spun on the wafer.

Several different braid structures may also be produced from the fibersas shown in FIGS. 11, 12 and 13. Braiding patterns are illustrated assingle lines along the length of the braids to better illustrate suchpatterns. The ends of the braids illustrate a cross section of thefibers used in the braids. A solid braid 1100 comprises multiple fibersbraided together in a solid structure. Each fiber is wound on a bobbin,and the process for making the braid is very similar to that used innormal textile braiding. A hollow braid 1200 comprises a ring of fibersbraided with a hollow core. No bobbins are positioned in the middle,resulting in the ring of adjacent fibers with no fibers in the middle ofthe structure. Finally, a low angle braid 1300 is formed by adjustingthe tension on the braid while the braiding process occurs.

Another example MEMS device utilizing such fibrous materials of thepresent invention is a torsional opto-mechanical scanner, such as thatshown in FIG. 14 at 1400. Scanner 1400 comprises a mirror 1410 coupledto an inner frame 1415 by a pair of coaxial opposed inner torsionalelements 1420 and 1425. The inner frame 1415 in turn is coupled to anouter frame 1430 by a pair of coaxial opposed outer torsional elements1435 and 1440 which are orthogonal to the inner torsional elements. Theaxes of both sets of torsional elements are co-planar in one embodiment.The arrangement of frames and torsional elements allow the mirror 1410to move about both axes of the torsional elements.

In one embodiment, the inner and outer frames 1415 and 1430 are formedusing biaxial arrays of micro-fibers, the torsional elements are formedusing braided fibers, and the central mirror 1410 is formed by as awoven fiber sheet such as that shown in FIG. 6. The torsion braids arehollow, and may have fibers in their cores for other purposes in variousembodiments. The braids are coupled to the frames by one of manydifferent mechanisms. In one embodiment, they are interlaced with theframes. Interlacing points between fibers in the structure providemechanical stability.

Scanner 1400 is a staggered frame configuration where the central mirror1410 is a plate of any desired symmetric shape, suspended by torsionelements at two opposite ends from a rectangular frame. The mirror maybe coated with aluminum, chrome, copper, silver or gold in varyingembodiments. The frame in turn is also suspended from a substrate bytorsion elements on opposite sides, however these elements lieorthogonal but in the same plane to those suspending the mirror element.Angular deformation of the torsion elements along their axes producescorresponding rotation of the central mirror plate along two orthogonalbut coplanar axes.

In one embodiment, carbon/graphite fiber micro-braids serve as thetorsional elements. Unlike MEMS scanners based on silicon and relatedmaterials, the torsional elements use graphite fibers braided ordouble-coiled into helical structures. Such structures transform thestresses within the torsional elements from shear into tension orcompression along individual fibers. Since such fibers are typically anorder of magnitude stronger under tensile or compressive stresses ascompared to shear stresses, these torsional elements can be madesignificantly stronger, lighter, and with tailorable stiffness, than anycurrently used monolithic material. Additionally, properties of thetorsion element can be also tailored through the dimensions of thebraid, and the braiding angle, the types of fibers used to construct thebraid. These enhancements and tailorability of the strength, stiffnessand density result in MEMS scanner devices capable of undergoing muchlarger angular deflections at very high resonance frequencies withoutfailure for an extended number of cycles. This results in scanners thatcan be adapted or tailored to meet specific scanning or opticalswitching applications.

In addition to the use of micro-braids as the torsional elements, therest of the scanner device i.e., the frames 1415 and 1430 and mirrorplate 1410 may also be composed of arrays of fibers, either as alignedarrays or fabrics. A fabric base is used in one embodiment as opposed torandom or aligned fiber arrays since fiber interlacing points in afabric make delamination difficult, resulting in better mechanicalproperties. Additionally, the micro-braids can be interlaced intofabrics easily facilitating fabrication and construction of the device.

Alignment of the fibers and braids within specific MEMS structures isalso easier with a fabric. In addition to the micro-braids in thetorsional elements, the woven fabric serves to enhance the mechanicalproperties of those structures in devices, such as the central mirrorplate and surrounding frame element that face significant deformationdue to inertial forces resulting from high frequency resonant operation.Deformation of the central mirror element results in inconsistentoptical scans, and deformation of the frame element in addition to themirror plate can cause the MEMS scanner to oscillate in modes other thanthe preferred mode of operation. A fabric base stiffens these elementsto minimize deformation during dynamic operation due to inertialeffects, while keeping the mass of these structures at a minimum. Simplyaltering the weave, fiber types, and amount and type of matriximpregnation in a fabric creates devices with different properties indifferent structures.

Given the above properties, single fine diameter fibers making up theindividual helices in the braid are capable of consistently undergoingbend radii of as little as 10 times their own diameters. Note thattensile stress level in the fiber walls is inversely proportional tobend radius. In fabricating the torsional elements, the initial fiberconfigurations will have local radii several times larger than they areultimately capable of, which means that the bending stresses will be farbelow the threshold for their failure. This allows the creation ofdynamic/moving MEMS devices that use torsional deflection (twistingmotions) to produce movement with exceptional torsional performance (byseveral times) compared to what is possible using existing MEMSmaterials.

In one embodiment torsional elements, are only a single braid consistingof only a few tens of fibers. In other embodiments, multiple layerbraids may be used.

The torsional strength and stiffness of tubular braids is affected bythe helix angle (i.e., the angle at which individual coils are wrappedaround to form a braid), and crimp angle (i.e., the angle formed atfiber cross-over points.) Additionally other factors such as the numberand diameter of the fibers making up the braid, the number of cross overpoints formed by the fibers, spacing between individual coils, and braiddiameter and length, along with the amount and type of impregnation inthe braid also affect the braid's strength and stiffness in tension.

Large diameter (100 μm) braids made with fine diameter (2-6 μm) fiberscoiled at low helix angles, and with few cross over points betweenfibers, and modest spacing between the coils, results in braids in whichthe fibers are initially stressed to a fraction of theircapabilities—that is bend radii of about 4 to 5 times the fiberdiameter. For example for a 100 μm diameter (R_(o)) braid made of 5 μmdiameter fibers coiled at a helix angle (α₀) of 45° the curvature of thefiber in the unstrained braid is given by:

$k = {\frac{\sin^{2}\alpha_{0}}{R_{0}} = {\frac{(1.414)^{2}}{100\mspace{14mu}{µm}} = \frac{1}{50}}}$

In further embodiments, anchoring is performed by splaying the fibers(beyond the flexure region) into a broad fan, which are then embeddedinto the mirror and frame structures. By expanding the braid in theseregions, the anchoring is increased by a factor essentially equal to thenumber of fibers in the fan. This solution may complicate thefabrication of devices in a microprocessing flow, but can be achievedfor at least small numbers of elements.

The majority of the fibers in the scanner 1400 (indicated by solid lines610) in FIG. 6 are chosen to control the stiffness and strength of thefabric, while some, such as a minor fraction of the fibers (indicated bydotted line 615) are electro-mechanically functional for actuation andor sensing. For example they may be ferromagnetic nickel or permalloyfibers that couple to a solenoid and cause the entire element to movedue to the magnetic interaction. For each scanner 1400, the weaveincorporates a single small diameter graphite fiber braid 620 serving asthe torsion bars.

In the embodiment where magnetic actuation is used to oscillate thescanner device, the density of permalloy fibers incorporated within thestructures is directly coupled to the strength of the solenoidactuators. The strength of the interaction depends on the total volumeof magnetic material that can be incorporated into the MEMS structure.Traditionally, these are electroplated onto structures of the MEMSdevice. In the case of fabrics, magnetically functional fibers 615, areincorporated with the oriented structural fibers or woven directly intothe fabric, integrating the actuation directly into the overall fabricor fiber array. Additionally, other types of fibers 615 can also beincorporated into MEMS elements creating a hybrid fibrous structure withsome fibers acting as integrated sensor elements. Sensing can beparticularly critical in optical switching applications. For example,the torsional braid 620 can be constructed with a piezoelectric core1510 as seen in FIG. 15, or by incorporating one or more fibers withpiezoelectric properties in the braid. Braid 620 is then used toactively sense the angular deflection of the mirror.

Fabrication methods for said torsional scanners utilizes techniques fromsemiconductor manufacturing and existing MEMS processing and may besimilar to that described earlier.

One fabrication process for said fibrous MEMS devices and scannersinvolves aligning fiber arrays or fabrics onto the surface of anoxidized silicon wafer and impregnating the fibers with a suitablematrix material. The matrix material may be selectively cured in certainareas to provide varying degrees of stiffness in different parts of thescanner devices. For example laser energy may be used to selectivelycure the structure making up the mirror and torsional elements to agreater extent to enhance their stiffness.

Localized curing of the matrix allows the tailoring of stiffness andstrength. In the scanner, the mirror element must be extremely stiff toavoid dynamic deformation during scanning. However, the torsional beamsmust only be stiff enough for the high frequency operation—anyadditional stiffness increases the power requirements. To address thesedisparate requirements, the matrix is selectively cured in differentparts of the scanner. In one embodiment, both a pulsed 1064 nm Nd:YAGlaser and a pulsed 308 nm XeCl excimer laser are used to selectivelycure the mirror element and supporting structures. The cure is adjustedfor the torsional elements. A pulsed laser (30 ns) will thermally affectonly the irradiated area; heat diffusion is almost purelyone-dimensional in this time regime. Fluences of 100 mJ/cm² are adequatewith 5-10 pulses per area. Under these conditions, a conventionalexcimer laser (50 W) can treat a 200 mm wafer area in well under aminute. In this way, a scanner device is produced where the mirrorelement has very high stiffness (modulus >150 GPa) while the torsionbars are elastic to undergo large shear deformations, but sufficientlystiff to resonate at high frequencies.

The torsional element braids in the fabric are aligned precisely toensure that they are totally contained within the lithographicallydefined regions in one embodiment. The torsion beams are maintaineddefect free, and it is important to avoid or at least minimize anyetching of the outer elements of the braids. In one embodiment, apre-etching of alignment slots in the silicon substrate is performed asshown in FIG. 16. The thicker braided yarns (which will form the torsionbeams) are separated from the other fine-diameter fibers and thencaptured in the slots. This then aligns the oriented fibers, or fabricto the substrate.

Carbon fibers etch well in oxygen and other plasmas used in conventionalMEMS and IC processing. The matrix and fibers used may also be etchedout in similar plasmas such as CF₄/H₂/O₂ or Cl₂ and argon. Elements ofthe silicon wafer are etched using standard wet etching chemistries.

A single axis scanner may also be fabricated incorporating the torsionalbraid and fabric mirror element in the same manner as the dual axisscanner

CONCLUSION

Fibrous micro-composite materials are formed from micro fibers. Thefibrous micro-composite materials are utilized as the basis for a newclass of MEMS. In addition to simple fiber composites andmicrolaminates, fibrous hollow and/or solid braids, can be used instructures where motion and restoring forces result from deflectionsinvolving torsion, plate bending and tensioned string or membranemotion. In some embodiments, these materials will enable simultaneoushigh operating frequencies, large amplitude displacements and orrotations, high reliability under cyclical stresses.

In one embodiment, fibrous elements are formed using high strength,micron and smaller scale fibers, such as carbon/graphite fibers, carbonnanotubes, fibrous single or multi-ply graphene sheets, or othermaterials having similar structural configurations. The type of fiberused may vary greatly. Some examples in addition to carbon fibers (andtheir penultimate carbon nanotubes), include but are not limited toglass fibers, Kevlar fibers, metal fibers (magnetic or electricallyconductive), etc.

One MEMS device formed using the micro-composite materials is an opticalscanner. The optical scanner has a mirror that is sufficiently stiff toresonate at high frequencies—approaching and eventually exceeding 10kHz, and is tough enough to undergo large elastic deformations in eitherbending or torsion so as to actuate the mirror element through angulardeflections approaching 40°. Fiber properties, fiber volume fraction,fiber orientation, resin materials and the fiber/resin interfacecharacteristics can all be adjusted to modify the properties of thescanner. Scanners may be optimized for stiffness and elasticity bycombining the stiffness and strength of one material with the elasticityof another.

The fibrous MEMS materials technology is also compatible with CMOSprocessing, which enables full system development with active controland sense circuitry on associated wafers. The anticipated commercialbenefits of the use of fibrous based materials for MEMS devices are farand wide reaching.

One of the largest commercial markets for high speed/large deflectionMEMS—as envisioned today—is the optical imaging and Telecommunicationspaces, specifically scanners and optical switches. Raster scanneddisplays could be substantially miniaturized as current performancelimitations are addressed, and the inherent size and mass produciblenature of MEMS scanners also allows for a dramatic reduction in the costand power consumption of these systems. Fibrous micro-composite MEMSscanners may be used in video display applications, optical crossconnects for telecommunications networks, spatial light modulators,laser printer and optical data storage heads, barcode scanners etc. Amyriad of other broad and niche applications exist for high performancescanners, such as endoscopic and confocal microscopes or spatial lightmodulators for use in laser printers, barcode scanners and opticalstorage heads. The small size and weight, low power consumption and lowcost of the MEMS scanners also translate to the same advantages in themicro display markets.

1. A micro-electrical-mechanical system comprising: a substrate; a fiberwithout optical functionality supported by the substrate; a structure,coupled to and supported by the fiber in a manner that enables separatemovement of the structure from the substrate.
 2. Themicro-electrical-mechanical system of claim 1 wherein the separatemovement of the structure is in a controlled path.
 3. Themicro-electrical-mechanical system of claim 1 wherein the fiber ismechanically coupled to both the structure and the substrate.
 4. Themicro-electrical-mechanical system of claim 1 wherein the fibercomprises a carbon fiber.
 5. The micro-electrical-mechanical system ofclaim 1 and further comprising an active element that causes motion ofthe fiber.
 6. A micro-electrical-mechanical system comprising: asubstrate; a carbon fiber having a first longitudinal portion supportedby the substrate and a second longitudinal portion free to move relativeto the substrate; a structure, mechanically coupled to the secondlongitudinal portion of the carbon fiber in a manner that supports thestructure and causes separate movement of the structure from thesubstrate.
 7. The micro-electrical-mechanical system of claim 6 whereinthe substrate comprises silicon.
 8. The micro-electrical-mechanicalsystem of claim 6 wherein the substrate comprises a semiconductor.
 9. Amicro-electrical-mechanical system comprising: a substrate; a carbonfiber having a length and width, and having a first length of the carbonfiber supported by the substrate and a second length of the carbon fiberfree to move relative to the substrate; a structure, mechanicallycoupled to the second length of the carbon fiber in a manner thatsupports the structure and causes separate movement of the structurefrom the substrate.
 10. The micro-electrical-mechanical system of claim9 wherein the structure is attached to an end of the second length ofthe carbon fiber.
 11. The micro-electrical-mechanical system of claim 9wherein the carbon fiber includes at least one of graphite fibers,carbon nanotubes and graphene sheets.